1. Field of the Invention
This invention relates to metal containers and their manufacturing methods and apparatuses.
2. Description of the Prior Art
A steel drum, for example, is made by joining the flanges formed at the edge of a cylindrical body sheet and an end sheet (top and/or bottom). After properly laying and temporarily fitting one flange over the other, the flanges of the body and end sheet are multifolded to form a seam. The conventional seams of metal containers come in such cross-sectional shapes as elliptical, egg-shaped, round-cornered rectangular, spiral, and their variations. At the center of an elliptical, egg-shaped, rectangular and other similar elongated seam, the extreme edges of the body and end plate flanges are hooked flat to each other. The inner portions of the flanges are tightly multifolded, one flange parallel to the other. In a spiral seam, on the other hand, each individual fold has a gradually varying radius. Because of the flat hooking at the center, the elongated seam like the elliptical one requires larger body and end plates than the spiral one, which results in higher production cost. In addition to this economic disadvantage, the seam of this type does not have uniform strength in all directions. Meanwhile, a spiral seam having folds of gradually varying radii is usually made up of two metal sheets. Since the forming groove on a seamling roll cannot have an opening wider than 180 degrees, the metal sheets leaving the forming groove are curved in conformity with the radius of curvature at the exit end of the roll. This results in an undesirable mixture of spiral and circular bends in the seam, or, otherwise, an undesirable gap near the seam center which, in turn, leads to a larger seam and, therefore, calls for the use of larger body and end sheets. A typical example of the conventional seaming methods is the one proposed by Wessely (which is also known as the Gallay's method), U.S. Pat. No. 3,425,381 - Re. 29,307, which comprises the following three steps that are carried out in succession:
(1) To curl, using a roll, the edge of the flange of an end sheet (top or bottom) until the same comes into substantially immediate facing relation with the flange of a body sheet (ref., Johnston, U.S. Pat. No. 3,160,312, and Compo, U.S. Pat. No. 2,337,452);
(2) To impart a relative displacement to the flanges of both end and body sheets (the known multifolding bending principle traditionally utilized in the formation of a five-fold seam); and
(3) To roll the flange edge of the body sheet through substantially 270 degrees, thereby interengaging the flange edges of both sheets to form a seven-fold seam (ref., Rheem, U.S. Pat. No. 2,169,395, and Defauw, United Kingdom Pat. No. 506,182).
According to this method combining the known fabrication steps, a seven-fold seam can be obtained by a continuous bending forming operation on a roll, without requiring the ultimate flattening of the seam. The disadvantage of this method is as follows: The end sheet flange is curved along the forming groove on a seaming roll to form the outline of a seam. Toward the end of the seaming operation, the flange edges of both sheets strike against the inner portions thereof to get deformed. It is when seaming is completed that the flange edges are interlocked to form the center of the seam. For this reason, the deformation of the flange edges forming the seam center does not take place in a stable fashion. The interlock and contact between the two flanges are neither stable nor uniform, leaving an undesirable gap in the seam.
Another known seaming method is that which was proposed by Coppens and Steen (known also as the Van Leer's method). This method disclosed in Netherlands Pat. Nos. 6,911,769 and 7,009,657, U.S. Pat. No. 3,736,893 and Japanese Patent No. 989,815 comprises the following three steps:
(1)' To bend the edge of the end flange about 180 degrees into a U-shaped bend with a first seaming roller having in cross-section a semi-circular groove of very small radius, and apply a sealing compound in the U-shaped bend (ref., W. R. Grace & Co., Dewey and Almy Technical Bulletin, Jan. 15, 1954 and Can Containers and Double Seam, The Can Society of Japan, 1965);
(2)' To form the body flange in which a widewall of said body merges into the end flange with a radius which is considerably larger than the radius of curvature at the junction of the end flange and an adjacent cylindrical portion of said end (ref., Kinney, U.S. Pat. No. 2,460,296, Johnston, U.S. Pat. No. 3,160,312 and Krummel, German Patent No. 217,070); and
(3)' To roll the flanges closely into each other with a seaming roller having, when viewed in cross-section, a semi-circular groove bottom in which the end flange is bent over more than 360 degrees and the body flange is bent over more than 270 degrees to form a seam connection having a closed spiral shaped cross-section.
This method provides a seam that has the following characteristics:
(4)' The seam wherein the flanges are in a spiral shaped winding of gradually changing radii and are in metal-to-metal contact throughout the entire cross-section of said seam connection (ref., Robinson, U.S. Pat. No. 1,389,900 and Krummel, German Patent No. 217,070);
(5)' The sealing compound resides exclusively in the center of the seam (ref. Krummel, German Patent No. 217,070, and Sebell, U.S. Pat. No. 2,197,439); and
(6)' The center of the seam is completely filled with the sealing compound under pressure. (The sealing compound is naturally kept under pressure since seaming is effected after the sealing compound has been filled and dried according to step (1)' described previously.)
This seaming method and the seam made by it has some shortcomings. Namely, the end enclosure flange forms the outline of a seam running along the semicircular groove on the seaming roll. Toward the end of the seaming operation, the hollow edge (U- or channel-shaped) of the end closure flange strikes against the body plate and gets deformed. The bending moment to cause this deformation is great in the curved portion of the U-shaped edge and the adjacent portion of the flange. The bending moment is also restrained by the semi-circularly shaped edge of the body flange. Therefore, the bend radius of the hollow edge increases, as a consequence of which the seaming operation completes without bending the straight portion of the hollow edge. Because of this, the pre-filled sealing compound must be elastic and plentiful enough to allow the hollow edge to undergo a deformation of any fashion and any magnitude. Besides, the unbent portion of the hollow edge does not stick fast to the opposite body flange, creating an undesirable gap in the seam.
Features of the spiral shaped seam and the method for making the same in this U.S. Pat. No. 3,736,893 are characterized in such manners that the seam has very much sealing compounds and the body flange edge is cored in the bent hollow edge of the end closure namely in the sealing compound present in it but cannot, however, shift with respect to the bent hollow edge during the seaming operation.
FIGS. 1-a through 1-c and FIGS. 2-a through 2-c are cross-sectional views respectively illustrating a five-fold seam and a seven-fold seam made by the method just described in three stages; i.e., (a) temporary interlocking (to start seaming), (b) a later stage of seaming, and (c) completion of seaming. Reference character 1 designates an end closure flange, 2 a body flange, R the semi-circular groove on a seaming roll, and 1' and 2' end and body plates proper contiguous to the end closure and body flanges, with a chain line indicating the center axis of the semi-circular groove.
FIG. 1-a and 2-a show the temporary interlocking stage with which a seaming operation begins. The end closure and body flanges are bent and folded together by the semi-circular groove R of the descending seaming roll.
FIG. 1-b shows a state in which the extreme edge of the body flange 2 has been bent by 180 degrees in a five-fold seam. FIG. 2-b shows a state in which the U-shaped edge of the end closure flange 1 in a seven-fold seam comes in contact with the body flange 2. In both figures, the extreme edge of the body flange 2 is cored in the U-shaped edge of the end closure flange 1. Coring begins with a given winding angle which depends, assuming that both flanges have the same thickness, upon the length of the protruding straight portion of the U-shaped edge, the relative positions of both flange edges in the temporarily interlocked condition, and the radius of curvature of the semi-circular groove R on the seaming roll, and ends with another given winding angle.
The bending-forming by the semi-circular groove on the seaming roll reduces the bend angle of the U-shaped edge to below 180 degrees. When the edge of the end closure flange 1 touches the body flange, the bend angle temporarily increases. Since the width of the U-shaped hollow edge is larger than the flange thickness, however, the bend angle of the U-shaped edge soon becomes smaller than 180 degrees. A comparison among FIGS. 1-a, 1-b and 1-c and among FIGS. 2-a, 2-b and 2-c clearly shows that the relative positions of both flange edges move as the seaming operation proceeds.
FIG. 1-b shows a state in which the extreme edge of the body flange 2 has been bent by 180 degrees. The seaming operation of a five-fold seam must have been completed in this state. As shown, however, the external side of the U-shaped edge of the end closure flange 1 is not in contact with the body flange 2, with the sealing compound not residing at the center of the formed seam. To avoid this, the U-shaped edge must have been deformed, striking against the body sheet proper 2', before the seaming operation reaches the state of FIG. 1-b. It is therefore indispensable to this method that the U-shaped edge of the end closure flange 1 gets deformed striking against the body sheet proper 2'. The force causing this deformation acts on a point at which the U-shaped edge touches the body sheet proper 2'. A maximum bending moment works on that point of the end closure flange 1 which rests at the exit end of the semi-circular groove R on the seaming roll. Therefore, the end closure flange 1 contiguous to the U-shaped edge undergoes the sharpest bend at the exit end of the semicircular groove R. A deforming force arises when the extreme edge of the end closure flange 1 is restrained by the semi-circularly bent body flange. Then, a maximum bending moment arises where the U-shaped edge touches the body sheet proper 2', whereupon the bend angle increases to open the U-shaped edge. As a result of all this, the seam shown in FIG. 1-c is formed. As illustrated, the upper half of the seam is circular, while the lower half is composed of a curve and a straight line. Particularly the end closure flange 1 forms two curves with gradually varying radii and two straight lines at the center of the seam. One of the straight lines remains unbent because the bending moment to open the U-shaped edge is smaller in the straight portion of the U-shaped edge than at the point where the U-shaped edge touches the body sheet proper 2'. This leaves a gap between the edge of the end closure flange and the opposite body flange, making it theoretically very difficult to bring both flanges into close contact with each other.
FIG. 2-b shows a state in which the U-shaped edge of the end closure flange 1 touches the body flange 2 in a seven-fold seam. Assuming that the under side of the end closure flange 1 gets deformed striking against the body sheet proper 2' before reaching the state shown in FIG. 2-b, the force acting on the point of contact exercises a maximum bending moment on both flanges 1 and 2 at the exit end of the semi-circular groove R on the seaming roll, thereby causing both flanges 1 and 2 to undergo the sharpest bend at the same exit end. Meanwhile, the force working on the point where the U-shaped edge of the end closure lange 1 touches the body flange 2 exercises a bending moment opposite to said bending moment on that point of both flanges 1 and 2 which rests at the exit end of the semi-circular groove R and a maximum bending moment at a point where the lower side of the end closure flange 1 touches the body sheet proper 2', thereby bending the end closure flange 1 in the seaming direction. This deformation temporarily reduces the opening of the U-shaped edge. Since the extreme edge of the end closure flange 1 is restrained by the internal side of the semi-circularly shaped body flange 2, however, the U-shaped edge opens again, whereupon the straight portion thereof is bent to form a seam shown in FIG. 2-c. The seam schematically sketched in FIG. 2-c has a more complex form in reality. While the upper half is circular, the lower half is composed of curves and straight lines. Especially, the end closure flange 1 consists of two curves with gradually varying radii at the center of the seam and two straight lines. The extreme edge of the end closure flange 1 is out of contact with the body flange 2, leaving a gap therebetween. The difference between the winding angles of both flange edges exceeds 180 degrees. If the straight portion of the U-shaped edge were shortened so that the winding angle difference should be kept within 180 degrees, the edge of the end closure flange would be restrained to a lesser extent by the internal side of the body flange, whereby the U-shaped edge would be deformed less to make said gap larger. If the under side of the end closure flange 1 did not touch the body plate proper 2' and get deformed in FIG. 2-b, the whole body flange 2 would be bent circularly, with the U-shaped edge of the end closure flange 1 only getting deformed upon contact with the body flange 2. Accordingly, the end closure flange 1 at the seam center would be formed like the case shown in FIG. 2-c to leave a still greater gap. This defect in the seam is due to the seaming method which forms a U-shaped edge before seaming, varies the radius of curvature of the U-shaped edge at the center of the seam in the later stage of the seaming operation, and, thereby, turns the straight protruding portion of the U-shaped edge. Besides, a large quanty of sealing compound elastic enough to withstand any deformation during seaming must be laid from within the U-shaped edge to the extreme edge of the end sheet in order that the center of the seam is completely filled with the sealing compound upon completion of seaming. Meanwhile, the center of the seam must be large enough to allow the straight protruding portion of the U-shaped edge to turn as the radius of curvature thereof varies. This spells a large seam diameter and, therefore, the requirement for larger body and metal plates. The extreme edge of the end closure flange, either straight or curved in shape, remains out of contact with the opposite body flange. Because of the need to form the U-shaped edge and fill and allow to dry a sealing compound therein before seaming, this method involves more processes, more equipment and higher production cost than the aforementioned method according to U.S. Pat. No. - Re. 29,307. Circularly formed with a large radius of curvature, the body flange is apt to get loose and disengaged from the temporarily interlocked end sheet. With the U-shaped edge deforming unstably and that side of the seam which does not contact the semi-circular groove forming unstable curves and straight lines during seaming, it is difficult to theoretically exactly set the length of both body and end closure flanges. It is therefore difficult to ensure the uniformity of seam form, especially that of the seam center structure. It is also difficult to attain a close contact between both flanges at the center of the seam, with the straight portion of the U-shaped edge remaining unbent or getting deformed into form a curved line.
The unstable deformation of the seam-forming metal sheets during the seaming operation and the unstable form of the seam itself are the defects common to the conventional seams and their forming methods. It has therefore been impracticable to theoretically exactly calculate and determine the length as well as the straight and curved portions of both flanges for temporary interlocking.